According to an example of an image forming apparatus, such as laser printer, a drum unit that holds a photosensitive drum is detachably mounted to a main body of the apparatus.
The photosensitive drum has a cylindrical hollow drum main body and a drum shaft extending along a central axis line of the drum main body. The drum shaft is held to a frame of a drum unit so that it cannot be rotated. The drum main body is rotatably supported to the drum shaft. A flange is fitted to one end portion of the drum main body and a drum gear is connected to the other end portion thereof. When driving force of a motor is input to the drum gear, the photosensitive drum (drum main body) is rotated in a predetermined direction.
A developing unit that holds a developing roller is mounted to the frame. When the developing unit is mounted to the frame, the developing roller is pressure-contacted to the drum main body. While being pressure-contacted to the drum main body, the developing roller is rotated in a reverse direction with respect to a rotating direction of the photosensitive drum so that a part pressure-contacted to the drum main body is moved in the same direction as a surface of the drum main body. As the photosensitive drum and the developing roller are rotated, toner is supplied to the surface of the drum main body from the developing roller and an electrostatic latent image formed on the surface of the drum main body is developed into a toner image. In addition, a transfer roller is opposed to the drum main body. While opposed to the transfer roller, the toner image carried on the surface of the drum main body is transferred to a sheet that is introduced between the transfer roller and the drum main body.
A rotational speed of the drum main body should be kept to be constant during an image forming operation. In other words, when the rotational speed of the drum main body is changed, the electrostatic latent image formed on the surface of the drum main body is expanded and contracted and the image (toner image) formed on the sheet is thus expanded and contracted depending on the change, so that a quality of an image formed on the sheet deteriorates.
The rotational speed of the drum main body may be changed due to a disturbance input to the drum main body during the image forming operation. For example, when a leading end of the sheet introduced between the drum main body and the transfer roller contacts the surface of the drum main body, the surface of the drum main body is pushed by the leading end of the sheet and the drum main body may be thus encouraged to rotate. Thereby, the rotational speed of the drum main body is changed (increased).
In order to suppress the change in the rotational speed of the drum main body, a braking member is pressed for the flange fitted to the drum main body from a direction following the drum shaft. Specifically, the braking member is provided for the flange at a position opposite to the direction following the drum shaft, and a press member for pressing the braking member toward the flange is mounted between the braking member and the frame of the drum unit. The braking member is pressure-contacted to the flange by the pressing force of the press member and frictional force is applied to the flange from the braking member, so that the change in the rotational speed of the drum main body is suppressed (for example, see JP-A-2007-316631).
When the braking member and the press member are provided, it is beneficial to increase the pressing force (elastic force) of the press member so as to suppress the change in the rotational speed of the drum main body (so as to exclude the effect of the disturbance input to the drum main body). However, when mounting the photosensitive drum to the drum unit, the braking member should be moved against the pressing force of the press member in a direction away from the frame of the drum unit during the mounting operation. Thus, when the pressing force of the press member is increased, much greater force is required to move the braking member. Therefore, the mounting operability of the photosensitive drum to the frame deteriorates.